In the manufacture of surfboards, body boards, wake boards and the like, unshaped polyurethane or polystyrene board blanks have been longitudinally split into halves, as shown in U.S. Published Patent Application No. 2008/0210137 to Cox, for adding strengthening members. Longitudinal splitting allows insertion of an elongated stringer or backbone between board halves which are joined together on either side of the stringer. It has been found that a surfboard with a carbon fiber stringer backbone is a stronger and faster surfboard than conventional surfboards having wood or plastic stringers, or no stringer.
The function of any stringer is to distribute forces on upper and lower surfaces of the board over the entire board. Using a carbon fiber stringer, forces on a board are distributed more effectively than with wood or plastic stringers. The unification of a carbon fiber stringer backbone with carbon fiber fins leads to improved force distribution from the fins to the entire board.
In board manufacturing, as taught in the published application to S. Cox, mentioned above, the board is shaped after joining halves together about the stringer. Shaping is done by trimming or adding material from or to the board blank to achieve a curved profile for the board, for example, a prow.
The stringer can be a linear member, typically a length of wood or plastic less than one centimeter wide, with variable thickness. Alternatively the stringer can have an I-beam shape, as shown in the published Cox application. In either case, the stringer is shaped together with halves of the board. For this reason, wood is a preferred material for shaping since wood grain remains tight and does not chip when worked with abrading tools that shape a board. Moreover, since the stringer edge is sometimes visible in a finished board, wood grain displays a nicer appearance than other materials.
The overall objective of stringers is achieving a fast, strong, lightweight board. An object of the invention is to devise a stringer that has improved flex patterns relative to carbon fiber stringers of the prior art, yet can have the favorable qualities of wood stringers. Improved flexing leads to better force distribution on a board and higher board speed.